U.S. patent number 7,919,366 [Application Number 12/576,938] was granted by the patent office on 2011-04-05 for laser crystallization method for amorphous semiconductor thin film.
This patent grant is currently assigned to The Japan Steel Works, Ltd., Osaka University. Invention is credited to Tatsumi Goto, Toshio Inami, Takahisa Jitsuno, Hideaki Kusama, Ryotaro Togashi, Keiu Tokumura.
United States Patent |
7,919,366 |
Jitsuno , et al. |
April 5, 2011 |
Laser crystallization method for amorphous semiconductor thin
film
Abstract
A laser crystallization method in which an amorphous silicon
thin film 2 formed on a substrate 1 is irradiated with a laser
beam, the method including the steps of providing the amorphous
silicon thin film 2 with an absorbent to form an absorbent layer 3
on the desired specific local areas of the amorphous silicon thin
film 2 and laser annealing for crystallizing the specific local
areas of the amorphous silicon thin film 2 by irradiating the
amorphous silicon thin film 2 including the specific local areas
with a semiconductor laser beam L having a specific wavelength
absorbable by the absorbent layer 3 and unabsorbable by the
amorphous silicon thin film 2 for heating the absorbent layer
3.
Inventors: |
Jitsuno; Takahisa (Osaka,
JP), Tokumura; Keiu (Osaka, JP), Togashi;
Ryotaro (Kanagawa, JP), Inami; Toshio (Kanagawa,
JP), Kusama; Hideaki (Kanagawa, JP), Goto;
Tatsumi (Kanagawa, JP) |
Assignee: |
Osaka University (Osaka,
JP)
The Japan Steel Works, Ltd. (Tokyo, JP)
|
Family
ID: |
42099253 |
Appl.
No.: |
12/576,938 |
Filed: |
October 9, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20100093182 A1 |
Apr 15, 2010 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 14, 2008 [JP] |
|
|
2008-265612 |
|
Current U.S.
Class: |
438/166; 117/10;
438/487; 117/904; 117/8 |
Current CPC
Class: |
H01L
21/02532 (20130101); H01L 21/02675 (20130101); Y10S
117/904 (20130101); H01L 21/268 (20130101) |
Current International
Class: |
H01L
21/268 (20060101) |
Field of
Search: |
;438/166,487
;117/8,10,904 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
63-142810 |
|
Jun 1988 |
|
JP |
|
2-7415 |
|
Jan 1990 |
|
JP |
|
2004-134577 |
|
Apr 2004 |
|
JP |
|
2005-191470 |
|
Jul 2005 |
|
JP |
|
Primary Examiner: Wilczewski; Mary
Attorney, Agent or Firm: Griffin & Szipl, P.C.
Claims
What is claimed is:
1. A laser crystallization method for crystallizing an amorphous
silicon thin film formed on a substrate using a laser beam, the
method comprising the steps of: applying a fluid comprising an
absorbent that absorbs a laser beam to a desired, specific local
area of the amorphous silicon thin film to form an absorbent layer
thereon, and laser annealing for crystallizing the specific local
area of the amorphous silicon thin film by irradiating the
amorphous silicon thin film including the specific local area with
a semiconductor laser beam having a specific wavelength absorbable
by the absorbent layer and unabsorbable by the amorphous silicon
thin film for heating the absorbent layer.
2. The laser crystallization method according to claim 1, wherein
in the step of laser annealing, the laser beam is a beam of an
infrared semiconductor laser, and the absorbent layer comprises an
infrared absorbent.
Description
This application claims priority from Japanese Patent Application
No. 2008-265612, filed Oct. 14, 2008, the entire disclosure of
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for laser crystallization
of an amorphous silicon thin film for forming a thin-film
transistor or the like.
2. Description of Related Art
Conventionally, thin-film transistors that are applied to a variety
of technical fields of TFT-LCDs, image sensors, touch-panel
sensors, and the like are formed in an amorphous silicon thin film
that is formed on a substrate; recently, however, various
techniques are employed to crystallize the amorphous silicon thin
film for the enhancement of response rate. The most widely used
crystallization method is a laser annealing method in which an
amorphous silicon thin film is irradiated with an excimer laser
beam having a wavelength of 308 nm or a second harmonic YAG laser
beam having a wavelength of 532 nm to crystallize the entire
surface of the amorphous silicon thin film.
FIG. 5 is an explanatory drawing showing a typical conventional
laser crystallization method. As shown in FIG. 5, the method of
laser beam irradiation is such that the line beam L of an excimer
laser is formed by constructing an optical system using an folding
mirror 10, slits 11 and 12, an annealer window 13, and like
components, and the substantially entire surface of an amorphous
silicon thin film 15 that is formed on a substrate 14 is irradiated
as the beam travels in the direction of the arrow X (scanning
irradiation). Excimer laser annealing apparatuses that give a line
beam (e.g., the line beam L) with a beam width (line beam width LW)
of 465 mm are produced in a commercial scale. Further, an
irradiation technique with which the entire target surface is
irradiated with a planar laser beam at once has also been
researched and developed.
In addition to the method that uses a laser of this type, Japanese
Unexamined Patent Publication No. 2004-134577, for example,
discloses a laser crystallization method in which a laser beam
absorption layer that is made from an alloy containing Cr, Mo, or
the like, or that has a layered structure thereof is formed on an
amorphous silicon thin film by vacuum deposition or sputtering, and
the absorption layer is then irradiated with a semiconductor laser
beam for heating and crystallizing the amorphous silicon thin
film.
However, the conventional laser crystallization methods are
problematic in that since the substantially entire surface of an
amorphous silicon thin film is irradiated with a laser beam in the
form of a line beam as the beam travels across the surface, heat
generated by the beam creates stress in the glass or silicon
substrate, resulting in cracking or warping.
To alleviate the thermal effect on the substrate, it is effective
to crystallize only the specific areas of an amorphous silicon thin
film necessary for forming a thin-film transistor or a like device;
however, it is difficult in a method in which the entire surface is
irradiated with a line beam of a laser, such as an excimer laser,
to crystallize only the specific areas of the amorphous silicon
thin film because the surface of the amorphous silicon thin film is
entirely crystallized (polysilicon formation) and then patterning
is performed by exposure/development according to a
photolithographic method.
SUMMARY OF THE INVENTION
Hence, a principal object of the present invention is to provide a
laser crystallization method that can crystallize only the desired,
specific local areas of an amorphous silicon thin film in which a
thin-film transistor, wiring, or the like is to be formed.
To achieve the aforementioned object, the present invention
provides a laser crystallization method in which an amorphous
silicon thin film formed on a substrate is irradiated with a laser
beam for crystallization. The method includes the steps of applying
a fluid that contains an absorbent that absorbs a laser beam to a
desired, specific local area of the amorphous silicon thin film to
form an absorbent layer thereon, and laser annealing for
crystallizing the specific local area of the amorphous silicon thin
film by irradiating the amorphous silicon thin film including the
specific local area with a semiconductor laser beam having a
specific wavelength absorbable by the absorbent layer and
unabsorbable by the amorphous silicon thin film for heating the
absorbent layer.
According to the present invention, an absorbent layer is printed
in desired areas on an amorphous silicon thin film that encompass
the specific local areas where a thin-film transistor, wiring, or
the like is to be formed and the amorphous silicon thin film
including the specific local areas is irradiated with a laser beam
that can be absorbed by the absorbent layer and cannot be absorbed
by the amorphous silicon thin film, so that only the absorbent
layer portion locally provided on the amorphous silicon thin film
is heated and crystallized and the amorphous silicon thin film
portion not provided with the absorbent layer is not heated, and
therefore the warping or cracking of the substrate can be
prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory drawing showing one step of the laser
crystallization method according to the present invention.
FIG. 2 is an explanatory drawing showing the step following that of
FIG. 1.
FIG. 3 is an explanatory drawing showing the step following that of
FIG. 2.
FIG. 4 shows an AFM picture of amorphous silicon crystallized by
the laser crystallization method according to the present
invention.
FIG. 5 is an explanatory drawing showing the principal part of a
conventional excimer laser annealing apparatus.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the laser crystallization method according to the
present invention is described below with reference to FIGS. 1 to
3.
First, as shown in FIG. 1, an amorphous silicon thin film 2 is
formed on a glass substrate 1 according to plasma-enhanced chemical
vapor deposition (PECVD), which is a conventional method of film
forming. A silicon substrate may serve as the substrate 1 for
reflection-type LCDs.
Next, as shown in FIG. 2, absorbent layers 3 are formed
(pattern-printed) by screen printing in specific local areas of the
amorphous silicon thin film 2 encompassing the areas where
thin-film transistors (TFTs) are to be formed. The absorbent layers
3 are printed over areas slightly larger than the areas where
thin-film transistors are to be formed. The method for printing the
absorbent layers 3 are preferably screen printing in terms of cost
and productivity. However, the method is not particularly limited
insofar as it allows the desired pattern to be readily printed, and
applicable examples include pad printing, ink-jet printing, and
like printing methods.
The material of the absorbent layers 3 is not particularly limited
insofar as it is printable and sufficiently absorbs a laser beam
when irradiated. The absorbent layers 3 can be formed by, for
example, applying to the substrate an absorbent-containing fluid
prepared by dispersing in a dispersion medium such as water or an
organic solvent an infrared absorbent or a near-infrared absorbent
that can absorb a semiconductor laser (wavelength: about 0.6 to 1.8
.mu.m) or an absorbent containing carbon black, amorphous carbon,
or the like that absorbs a laser of any wavelength, or by partially
dissolving such an absorbent in such a dispersion medium. Various
different organic and inorganic powders are commercially available
that can served as the infrared absorbent or the near-infrared
absorbent. Carbon black ink is usable as the carbon black
dispersion, and India ink is usable as the amorphous carbon
dispersion. Once the aforementioned absorbent-containing fluid is
applied, the absorbent layers 3 are thermally dried or
air-dried.
The thickness of the absorbent layers 3 applied is varied depending
on the type of the material of the absorbent layers 3, the output
of a laser beam, and other factors, but an example may be 0.1 to
0.8 .mu.m when a carbon dispersion is used.
The absorbent layers 3 are cured by a method suitable according to
the type of absorbent, and generally cured by air-drying.
A laser beam for use is a beam that cannot be absorbed by the
amorphous silicon thin film 2. The laser beam is of a semiconductor
laser, which costs lower than an excimer laser or a YAG laser.
Since the amorphous silicon thin film 2 has an absorption maximum
between 400 and near 550 nm and does not absorb a beam having a
wavelength of 700 nm or greater, the laser beam L for irradiation
is configured to have a wavelength of 700 nm or greater and,
therefore, an infrared semiconductor laser can be used as a
preferable laser beam source. A near-infrared semiconductor laser
and a far-infrared semiconductor laser are both usable. The
infrared semiconductor laser may use, for example, a
continuous-wave laser beam (i.e., continuous light) having a
maximum energy of 4 W and an irradiation energy density of 2.5 to
3.5 J/cm.sup.2.
After the absorbent layers 3 are formed as described above, the
laser beam L is emitted from a laser oscillator 4 as shown in FIG.
3. A method in which a line beam created by constructing an optical
system, as in a conventional method, is emitted such that the beam
travels across the surface for irradiation (scanning irradiation)
may be employed as the irradiation method. A known line
beam-emitting laser oscillator that uses, for example, a rod lens,
a cylindrical lens, a Powell lens, or the like may be used as the
laser oscillator for producing a line beam. An irradiation
technique with which the entire target surface is irradiated with a
planar laser beam at once may be employed in place of the
irradiation with a line beam. The oscillator 4 may be moved in the
arrowed direction in FIG. 3 while the irradiation with the laser
beam L is performed. Alternatively, the substrate 1 may be moved in
the opposite direction.
The substantially entire surface of the amorphous silicon thin film
2 including the specific local areas where the absorbent layers 3
are provided may be irradiated with the laser beam L.
When the laser beam L is emitted in the above-described manner,
only the absorbent layers 3 absorb the laser beam L while the
amorphous silicon thin film 2 and the glass substrate 1 transmit
the laser beam L. The energy absorbed by the absorbent layers 3 is
radiated as heat, thereby heating and annealing the amorphous
silicon thin film 2 disposed under the absorbent layers 3.
Therefore, only the specific local areas of the amorphous silicon
thin film 2 on which the absorbent layers 3 are provided are
crystallized.
As described above, the proportion of the crystallized portions
relative to the entire amorphous silicon thin film 2 is small and,
therefore, the total amount of heat generated is substantially
reduced in comparison with conventional methods in which the entire
amorphous silicon thin film is crystallized, and the warping or
cracking of the substrate 1 can thus be prevented.
According to the present invention, thermal effects on the
substrate 1 are alleviated, thereby allowing the extent of
crystallization (for example, particle size) to be controlled by
the amount of heat generated, and therefore, the extent of
crystallization can be more liberally controlled and the efficiency
of a thin-film transistor, such as carrier mobility, can be
improved. Accordingly, the applicability of a laser crystallization
method to TFT liquid-crystal panels is broadened, and the advantage
of a crystallization method that uses a low-cost semiconductor
laser can be exercised to the greatest extent.
For a working example of the present invention, an amorphous
silicon thin film having a thickness of 50 nm was formed on a glass
substrate using plasma-enhanced chemical vapor deposition, carbon
black ink was suitably applied to the amorphous silicon thin film
and dried at ordinary temperatures for 10 minutes, the amorphous
silicon thin film was irradiated with an infrared laser beam having
a diameter 0.5 mm, an output of 12 W, and an output density of 6.2
kW/cm.sup.2 while moving the scanning stage at a rate of 310 mm/sec
to crystallize the amorphous silicon thin film. An infrared
semiconductor laser oscillator DuO (manufactured by Coherent Japan)
was used. FIG. 4 shows an AFM picture of a portion where
crystallization was performed. When scanning irradiation with a
laser beam is carried out, there are portions where a laser beam is
given multiple times. However, the picture provided in FIG. 4 shows
that the portions that received multiple laser beam irradiation
cannot be distinguished from the portions where no multiple
irradiation was given, and uniform crystallization was attained
with a particle size of about 10 to 20 nm.
* * * * *